The present invention relates to an ultrasonic measuring device in which two ultrasonic transducers are arranged on both sides of a pipe in which a fluid to be measured flows, one of the ultrasonic transducers transmits an ultrasonic wave towards and through the fluid, and the other ultrasonic transducer receives the ultrasonic wave which has passed through the fluid.
As shown in FIG. 1, an ultrasonic measuring device such as an ultrasonic flow meter has two detectors (or ultrasonic transducers) D and D'. The detectors D and D' are arranged on the upstream side and the downstream side of a flow of fluid to be measured and the ultrasonic wave is propagated as indicated in the figure. The flow rate of the fluid is measured utilizing the fact that there is a time difference between a period of time t.sub.1 required for the ultrasonic wave to propagate from the upstream side to the downstream side and a period of time t.sub.2 required for it to propagate from the downstream side to the upstream side when the fluid is flowing and the magnitude of the difference is related to the flow rate of the fluid.
FIG. 2 is a perspective view of a conventional ultrasonic measuring device of this general type. In FIG. 2, reference numeral 1 designates a pipe in which a fluid at high temperature to be measured flows. An upstream detector 3 and a downstream detector 4 are mounted on opposite sides of the pipe 1, and the detection signals of the detectors are transmitted to a measuring circuit which is located remotely therefrom. For this purpose, coaxial cables which are designed to be used at room temperature are employed. However, since the heat resisting temperature of typical coaxial cables is limited to 100.degree. C. or lower, heat resistant wires 6 and 7 such as MI cables are used in the vicinity of the pipe 1 which is at a high temperature, for instance, 100.degree. C. to 500.degree. C. The heat resistant cable 6 or 7 is covered with a metal sheath. Therefore, it has a low flexibility, and it is heavy and expensive. Accordingly, the cables 6 and 7 are used only in limited lengths. A relay terminal box 8 is set at a place where the ambient temperature is not affected by the high temperature of the pipe 1. The ends of the heat resistant cables 6 and 7 are connected to the relay terminal box 8 from which ordinary coaxial cables 9 and 10 extend to a measuring circuit (not shown).
The metal sheath of the above-described heat resistant cable 6 or 7 is filled with insulating powder such as alumina. Therefore, it has a low flexibility, is rigid and heavy. Accordingly, the heat resistant cable suffers from the following difficulties: (a) Because of its low flexibility, wiring work with the cable is difficult. (b) If after being coupled to the frame of the detector during manufacturing the heat resistant cable is shipped, its package is bulky and is not suitable for transportation. (c) If, during installation or removal or during the maintenance and inspection the heat resistant cable is accidentally called by the operator's foot, for instance, the connection to the detector is liable to be damaged because of the rigidity of the cable. (d) Since the heat resistant cable has a low flexibility and is expensive, it is not suitable for use over long distances. Accordingly, the length of the heat resistant cable is limited, and the heat resistant cable must be coupled through an ordinary cable to the measuring circuit. For this purpose, it is necessary to provide a relay terminal box.
With an ultrasonic flow meter of this type, the flow rate of a fluid flowing in a pipe can be measured merely by mounting the detectors on the pipe. That is, it is unnecessary to bring the detectors directly into contact with the fluid to measure the flow rate. Accordingly, the ultrasonic flow meter is advantageous in that it can be used with a pipe after the pipe has been installed. Therefore, the detectors are often mounted on pipes and removed therefrom in a plant. However, the working temperatures in such situations are often very high or the working area may be contaminated with radiation making it dangerous to use the detectors. Accordingly, it is required that the ultrasonic measuring device meet the following requirements: (a) The construction thereof must be such that the device can be mounted on and removed from a pipe quickly with no intricate adjustment. (b) The construction must be such that the maintenance of the device can be easily performed. (c) In the case where a fluid to be measured is at high temperature (100.degree.-500.degree. C.) or it is at extremely low temperature (as in the case of liquid nitrogen or liquid hydrogen), the operator must not be exposed to the high temperature or extremely low temperature when he connects or disconnects the lead wires of the ultrasonic transducers.
Accordingly, an object of the invention is to provide an ultrasonic measuring device in which the above-described difficulties accompanying a conventional ultrasonic measuring device and which satisfies all the above-described requirements.